In this paper l deal with the substantiva! theory of space and time and I examine its consequences with respect to the problem of the differentiation of indiscernible particulars (the so called "problem of individuation"). I distinguish two versions: the "standard" theory and the "substratist" theory. According to the "standard" theory, space and time ( or spacetime) are substances made of points and instants (or point-instants) which hold a contingent relation with the physical substances which occupy those points and instants. (...) According to the "substratist" theory, which may be attributed to D.M. Armstrong, space and time ( or spacetime) are also substances made of points and instants (or point-instants), but those points and instants hold a necessary relation with the physical objects which "occupy" them. This is so because those points (or point-instants) are, as substrata, essential constituents of physical objects, which support and hold together their properties. I argue that in these theories we cannot appeal to the difference in spaciotemporal position in order to ground the numerical diversity of indiscernible particulars and, in this sense, they dont allow us to use space and time as "principies of individuation". (shrink)